Secondly, helps structure your thinking around a UI that sits atop 'data services'Quite a lot can be accomplished with Xaml markup - I think it would be interesting to see just how complex an 'application' could be created in a single Xaml file, using only databinding (and whatever server-side services you needed). WCF/REST could play a role (Syndication services... where's that link?).

The sandbox that loose Xaml sits in does present some problems... you can only load files from the origin, so no referencing RSS feeds from other servers. A middle-man pass-through (or other side-step) could get around that, however. It would seem trivial to create a fairly neat Gmail (or other service) reader; basically anything that publishes RSS (or any Xml, really). What is not clear to me just yet is whether it would be do-able to write a complete mail client, for example, following the same principles...

p.s. It's interesting to see how the databinding syntax changed, from "*Bind(XPath=author)" to "{Binding XPath=description}". Notice also how TextContent → TextXmlDataSource → XmlDataProviderID → NameFlowPanel → FlowDocument (?)Text → TextBlockI think it's kinda cool to see the tools evolve like that - same now with Silverlight...